Systems and methods for controlling multi-level diode-clamped inverters using space vector pulse width modulation (SVPWM)

ABSTRACT

Control systems for a multi-level diode-clamped inverter and corresponding methods include a processor and a digital logic circuit forming a hybrid controller. The processor identifies sector and region locations based on a sampled reference voltage vector V* and angle θ e *. The processor then selects predefined switching sequences and pre-calculated turn-on time values based on the identified sector and region locations. The digital logic circuit generates PWM switching signals for driving power transistors of a multi-level diode-clamped inverter based on the turn-on time values and the selected switching sequences. The control system takes care of the existing capacitor voltage balancing issues of multi-level diode-clamped inverters while supplying both active and reactive power to an IT load. Using the control system, one can generate a symmetrical PWM signal that fully covers the linear under-modulation region.

BACKGROUND

The present disclosure generally relates to Space Vector PWM (SVPWM)control for multi-level diode-clamped inverters for various applicationsincluding Data Center (DC) applications.

DCs are one of the largest and fastest growing consumers of electricityin the world. In 2013, DCs in the U.S. consumed an estimated 91 billionkWhr of electricity, which is enough electricity to power all thehouseholds in New York City twice over. And the DCs in the U.S. are ontrack to consume an estimated 140 billion kWhr by 2020.

A Multi-level Medium Voltage Data Center Static Synchronous Compensator(DCSTATCOM) or a Multi-level Medium Voltage Uninterruptable Power Supply(MVUPS) with battery energy storage may be employed to ensure a reliablesupply of power to DCs. DCSTATCOM or MVUPS connected in a shuntconfiguration at a point of common coupling (PCC) to a data center (DC)load, enables independent simultaneous control capability of both active(to provide UPS functionality, grid energy storage, and peak demand loadsupply) and reactive (to provide Power Factor (PF) correction, gridvoltage stiffness, and transient stabilizer functions) powercompensation for both DC and grid stabilization.

Multi-level voltage-fed PWM inverters are showing popularity inmulti-megawatt DCSTATCOM or MVUPS applications due to easy sharing oflarge voltages between the series-connected IGBT devices and improvementof harmonic quality at the output compared to the existing two-levelinverters with transformer systems.

SUMMARY

In aspects, the present disclosure features a control system for amulti-level inverter. The control system includes a digital logiccircuit, a digital up/down counter, a processor, and memory. The digitallogic circuit includes digital logic comparators, which include a firstcomparator and a second comparator, inverters, which include a firstinverter, coupled to respective outputs of respective comparators, andAND gates including an AND gate having a first input and a second input.The first input is coupled to the output of the first inverter and thesecond input is coupled to the output of the second comparator. Thecontrol system further includes a digital up/down counter coupled tofirst inputs of the comparators. The up/down counter counts from 0 toTs/2 and then from Ts/2 to 0 where Ts is the sampling period.

The control system further includes a processor and memory that identifya sector location based on an actual angle of a reference voltagevector, convert the actual angle into a converted angle located in afirst sector, identify a reference region location based on themagnitude of the reference voltage vector and the converted angle in thefirst sector, select a switching sequence and turn-on time values basedon the corresponding actual region location and actual sector, andtransmit turn-on signal values to second inputs of the plurality ofcomparators to generate switching signals for IGBT drivers of amulti-level inverter.

In aspects, the digital logic circuit is a Field Programmable Gate Array(FPGA) or an Application Specific Integrated Circuit (ASIC). In aspects,the processor is a digital signal processor (DSP).

In aspects, the number of the comparators and the number of theinverters is one less than the number of levels of the multi-levelinverter. In aspects, the number of the AND gates is one less than thenumber of the comparators.

In aspects, the processor and memory are further configured to convertthe reference voltage vector and the converted angle into X and Ycoordinate point values in the first sector, and identify a regionlocation based on the X and Y coordinate point values.

In aspects, the multi-level inverter is a five-level inverter, thecomparators further include a third comparator and a fourth comparator,the inverters further include a second inverter, a third inverter, and afourth inverter, the AND gates further include a second AND gate and athird AND gate, a first input of the second AND gate is coupled to theoutput of the second inverter and a second input of the second AND gateis coupled to the output of the third comparator, a first input of thethird AND gate is coupled to the output of the third inverter and asecond input of the third AND gate is coupled to the output of thefourth comparator, and the output of the first comparator, the outputsof the plurality of AND gates, and the output of the fourth inverterprovide the switching signals, which are transmitted to gate drivers fordriving power transistors of the multi-level inverter.

In aspects, the multi-level inverter is a four-level inverter, thecomparators further include a third comparator, the inverters furtherinclude a second inverter and a third inverter, the plurality of ANDgates further include a second AND gate, a first input of the second ANDgate is coupled to the output of the second inverter and a second inputof the second AND gate is coupled to the output of the third comparator,and the output of the first comparator, the outputs of the plurality ofAND gates, and the output of the third inverter provide the switchingsignals.

In aspects, identifying a region location includes comparing the X and Ycoordinate point values to segments of triangles, which representregions, in a vector space.

In aspects, the turn-on time values and switching sequence arepredetermined for each sector and region, and stored in a look-up tablestored in the memory.

In aspects, the present disclosure features a method of controlling amulti-level inverter. The method includes identifying a sector locationbased on an actual angle of a reference voltage vector, converting theactual angle into a converted angle located in a first sector,identifying a region location based on the magnitude of the referencevoltage vector and the converted angle in the first sector, selecting aswitching sequence and turn-on signal values based on the correspondingregion location in actual sector of reference voltage vector,transmitting turn-on time values to second inputs of the comparators togenerate switching signals, which are transmitted to gate drivers fordriving power transistors of the multi-level inverter, comparing each ofthe turn-on signals to a digital up/down counter signal to obtaincomparison signals including a first comparison signal and a secondcomparison signal, inverting the comparison signals to obtain invertedsignals including a first inverted signal, and performing a logical ANDoperation on the first inverted signal and the second comparison signalto obtain a switching signal for a corresponding driver that drives apower transistor of a multi-level inverter.

In aspects, the method further includes converting the reference voltagevector and the converted angle into X and Y coordinate point values, andidentifying a region location based on the X and Y coordinate pointvalues.

In aspects, identifying a region location includes comparing the X and Ycoordinate point values to segments of triangles, which representregions, in a vector space.

In aspects, the turn-on time values and switching sequences arepredetermined for each sector and region, and stored in a look-up tablestored in memory.

In aspects, the first comparison signal is a first gate drive switchingsignal (to generate either P₂ or N₂ switching states), the logical ANDoperation is performed on the first inverted signal and the secondcomparison signal to obtain a second switching signal (to generateeither P₁ or N₁ switching states), the comparison signals furtherinclude a third comparison signal and a fourth comparison signal, theinverted signals further include a second inverted signal, a thirdinverted signal, and a fourth inverted signal, the method furtherincludes performing a second logical AND operation on the secondinverted signal and the third comparison signal to obtain a thirdswitching signal (to generate O switching states), and performing asecond logical AND operation on the second inverted signal and the thirdcomparison signal to obtain a fourth switching signal (to generateeither N₁ or P₁ switching states), and the fourth inverted signal is afifth switching signal (to generate either N₂ or P₂ switching states).

In aspects, the present disclosure features an energy storage systemincluding an energy storage device, a DC-DC converter coupled to theenergy storage device, a multi-level inverter coupled to the DC-DCconverter, and a multi-level inverter controller coupled to themulti-level inverter. The multi-level inverter controller includes adigital logic circuit. The digital logic circuit includes comparatorsincluding a first comparator and a second comparator, inverters,including a first inverter, coupled to respective outputs of respectivecomparators, and AND gates including a first AND gate having a firstinput and a second input. The first input is coupled to the output ofthe first inverter and the second input is coupled to the output of thesecond comparator. The multi-level inverter controller further includesa counter coupled to first inputs of the plurality of comparators and aprocessor and memory. The processor and memory identify a sectorlocation based on an actual angle of a reference voltage vector, convertthe actual angle into a converted angle located in a first sector,identify a region location based on the magnitude of the referencevoltage vector and the converted angle in the first sector, select aswitching sequence and turn-on signal values based on the correspondingregion and actual reference voltage vector location, and transmitturn-on signal values to second inputs of the plurality of comparatorsto generate switching signals, which are transmitted to gate drivers fordriving power transistors of the multi-level inverter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram of a data center power supply systemaccording to embodiments of the present disclosure;

FIG. 2 is a circuit diagram of a three-phase five-level diode-clampedinverter according to embodiments of the present disclosure;

FIGS. 3A and 3B show a space vector diagram illustrating space voltagevectors of a three-phase five-level inverter according to embodiments ofthe present disclosure;

FIG. 4 is a space vector diagram illustrating space vectors indicatingregions and switching times according to embodiments of the presentdisclosure;

FIG. 5 is a space vector diagram illustrating switching states andswitching times of region 1 and sectors A-F for an example modeaccording to embodiments of the present disclosure;

FIG. 6 is a waveform diagram illustrating switching states for one phaseof sectors A-F and region 1;

FIG. 7 is a waveform diagram illustrating a sequence of switching statesof the three phases in regions 1-4 of sector A according to embodimentsof the present disclosure;

FIGS. 8-12 are waveform diagrams illustrating generation of turn-on timesignals and switching logic signals according to embodiments of thepresent disclosure;

FIG. 13 is a graphical diagram illustrating turn-on time values for onephase according to embodiments of the present disclosure;

FIG. 14 is a digital logic circuit for generating switching logicsignals to drive power transistors of a multi-level diode-clampedinverter according to embodiments of the present disclosure;

FIG. 15 is a flow diagram of a method of identifying region and sectorlocations of a voltage vector according to embodiments of the presentdisclosure;

FIG. 16 is a SVPWM controller for a five-level diode-clamped inverteraccording to embodiments of the present disclosure; and

FIG. 17 is a flow diagram of a method of controlling a multi-levelinverter according to embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure relates to a processor and digital logiccircuit-based hybrid controller and its implementation of SVPWM controlstrategies for multi-level diode-clamped inverters, e.g., multi-leveldiode-clamped inverters for Multi-level Medium Voltage Data CenterStatic Synchronous Compensator (DCSTATCOM) or Multi-level Medium VoltageUninterruptable Power Supply (MVUPS) with battery energy storage for adata center (DC) load connected at a medium voltage (MV) level. MVoperation reduces overall losses of DC components and hence improvesoverall efficiency of the system.

A control system of the present disclosure includes a digital logiccircuit, such as a Field Programmable Gate Array (FPGA), and aprocessor, such as a digital signal processor (DSP) or a microprocessor.The processor samples a reference voltage vector V* and an angle θ_(e)*,and identifies a sector and region based on the sampled referencevoltage vector V* and angle θ_(e)*. The processor then selectspredefined switching sequences and pre-calculated turn-on time valuesbased on the identified sector and region location of the referencevoltage vector V*. The digital logic circuit generates PWM switchingsignals for driving power transistors of a multi-level diode-clampedinverter based on the turn-on time values and the selected switchingsequences.

For multi-level inverters, the SVPWM control strategy is more suitablein comparison to sinusoidal PWM as the SVPWM control strategy offerssignificant flexibility to synthesize switching sequences of waveformsand is suitable for digital implementation by a processor, e.g., a DSP,and a digital logic circuit, e.g., an FPGA, forming a hybrid controller.

In SVPWM, the inverter voltage vectors, which correspond to the apexesof the triangle, which includes the reference voltage vector, aregenerally selected to minimize harmonics at the output in comparison tosinusoidal PWM. SVPWM also provides larger under modulation range thatextends the modulation factor to 90.7% from the traditional value of78.5% in sinusoidal PWM.

In the control systems of the present disclosure, a hybrid controller,which includes a processor and a digital logic circuit, is utilized toimplement various control blocks to carry out the SVPWM controlstrategy. The PWM signal generation task for providing PWM switchingsignals to the gate driver for driving a power transistor is carried outby a digital logic circuit, e.g., an FPGA. The remaining tasks areperformed by a processor, e.g., a DSP. Therefore, a less expensivehybrid processor and digital logic controller is used to implement anoverall complex control strategy. Also, the control tasks are dividedbetween a processor and a digital logic circuit to achieve a fastertransient response at lower cost.

The systems and methods of the present disclosure may be applied to aMulti-level Medium Voltage Data Center Static Synchronous Compensator(DCSTATCOM) or Multi-level Medium Voltage Uninterruptable Power Supply(MVUPS), as described in U.S. application Ser. No. 14/481,904, entitled“Multi-level Medium Voltage Data Center Static Synchronous Compensator(DCSTATCOM) for Active and Reactive Power Control of Data Centersconnected with Grid Energy Storage and Smart Green Distributed EnergySources”, filed on Sep. 9, 2014, and U.S. application Ser. No.14/594,073, entitled “Transformerless Multi-level Medium VoltageUninterruptable Power Supply (UPS) System”, filed on Jan. 9, 2015, eachof which are incorporated herein by reference in their entireties.

FIG. 1 is a system block diagram of a DCSTATCOM or MVUPS topology with atransfer switch 112 connected between a utility and a generator 110, astatic transfer switch 114, a transformer 116, and a DC IT load 118. TheDCSTATCOM or MVUPS systems include a battery energy storage block 120, abattery management system (BMS) controller 130, a bi-directionalmulti-level (ML) DC-DC converter 122, a DC-DC converter controller 124,a ML inverter 124 outputting medium voltage AC (V_(INV)) at the inverteroutput, and an SVPWM inverter controller 134 for controlling the MLinverter 124. As described in more detail below, the SVPWM invertercontroller 134 includes a processor and a digital logic circuit forgenerating PWM switching signals, which are applied to driver circuits(not shown) for driving power transistors (not shown) of the ML inverter124.

FIG. 2 is a circuit block diagram of a five-level diode-clampedinverter, which may be used as the ML inverter 124, which converts DCvoltage V_(DC) output from the converter 122 to three-phase AC voltageV_(AC). The five-level inverter includes power transistors or switchesS_(1U)-S_(8U), S_(1V)-S_(8V), and S_(1W)-S_(8W), and diodes connectedtogether in a diode-clamped configuration to generate three phases U, V,and W of an AC voltage V_(AC). State O represents neutral pointbalancing so that the average current injected at O should be zero.States P1 and P2 represent positive bus voltages. States N1 and N2represent negative bus voltages.

Switches S_(1U)-S_(8U), S_(1V)-S_(8V), and S_(1W)-S_(8W) may be powertransistors, such as IGBTs. IGBTs allow for higher voltages or currentsand higher switching frequencies. The five-level inverter illustrated inFIG. 2 allows for sharing of the high voltage among the switchesS_(1U)-S_(8U), S_(1V)-S_(8V), and S_(1W)-S_(8W), and reduces harmonicdistortion.

The complexity of an inverter control system increases as the inverterlevel increases from three to five or above. A three-phase five-leveldiode-clamped inverter illustrated in FIG. 2 is complex due to a highnumber of switching states, i.e., 5³=125 switching states, in comparisonto a three-phase three-level inverter with a lower number of switchingstates, i.e., 3³=27 switching states. The inverter must have a very fastresponse time (in micro-seconds) to have appropriate control operationand safety aspects for IGBT devices. Also, the SVPWM strategy needs toperform many on-line calculations due to its large number of switchingstates (e.g., 125 for a five-level inverter) and large number ofoperating regions/triangles (e.g., 96 for a five-level inverter).Therefore, in embodiments, the control systems of the present disclosureincorporate a digital logic circuit, such as an FPGA, to perform partiallogic functions. The digital logic circuit may form part of a hybridcontroller, which includes a processor and the digital logic circuit.This hybrid controller is well suited to implement SVPWM control becauseit allows for much larger bandwidth control and provides faster responsetimes.

The switching states of the five-level inverter are summarized in Table1, where X is one of the phases U, V, and W; and P₂ (+V_(DC)/2), P₁(+V_(DC)/4), O (0 V_(AC)), N₁ (−V_(DC)/4), and N₂ (−V_(DC)/2) are DC-buspoints as shown in FIG. 2.

TABLE 1 Switching States S_(1X) S_(2X) S_(3X) S_(4X) S_(5X) S_(6X)S_(7X) S_(8X) V_(XO) P₂ ON ON ON ON OFF OFF OFF OFF +V_(DC)/2 P₁ OFF ONON ON ON OFF OFF OFF +V_(DC)/4 O OFF OFF ON ON ON ON OFF OFF 0 N₁ OFFOFF OFF ON ON ON ON OFF −V_(DC)/4 N₂ OFF OFF OFF OFF ON ON ON ON−V_(DC)/2

Referring to Table 1, in conjunction with FIG. 2, phase U, for example,is in state P2 (positive bus voltage) when the switches S_(1U)-S_(4U)are closed or “ON”, and switches S_(5U)-S_(8U) are open or “OFF”. Thephase V is in state P1 (positive bus voltage that is less than P2) whenswitches S_(1U) and S_(6U)-S_(8U) are open or OFF, and switchesS_(2U)-S_(5U) are closed or ON. The phase U is in state O when switchesS_(1U), S_(2U), S_(7U), and S_(8U) are open or OFF and switchesS_(3U)-S_(6U) are closed or ON.

The phase U is in state N1, which corresponds to a negative bus voltagethat is greater than a negative bus voltage that corresponds to stateN2, when switches S_(1U)-S_(3U) and S_(8U) are turned off (i.e., open)and switches S_(4U)-S_(7U) are turned on (i.e., closed). The phase U isin state N2, which corresponds to a negative bus voltage that is lessthan the negative bus voltage that corresponds to state N1, whenswitches S_(1U)-S_(4U) are turned off (i.e., open) and switchesS_(5U)-S_(8U) are turned on (i.e., closed).

FIGS. 3A and 3B show a space vector diagram illustrating space voltagevectors of a five-level diode-clamped inverter with 125 switching statesand 96 operational triangles. There are 120 active switching states andthe remaining 5 are zero states (P₂P₂P₂, P₁P₁P₁, OOO, N₁N₁N₁, N₂N₂N₂)that lie at the origin. FIGS. 3A and 3B also show a hexagon having sixsectors—sectors A-F, and each sector has sixteen regions—regions 1-16,giving altogether 96 regions of operation.

In embodiments, the operation of a multi-level inverter, such as themulti-level inverter of FIG. 2, may be divided into multiple modes. Asshown in FIGS. 3A and 3B, the operation is divided into four modes—modes1-4. In Mode 1 (301), the command voltage vector V* trajectory coversregion 1 of all six sectors, e.g., sectors A-F. In Mode 2 (302), thecommand voltage vector V* trajectory covers regions 2, 3, and 4 of allsectors. In Mode 3 (303), the command voltage vector V* trajectorycovers regions 5, 6, 7, 8, and 9 of all sectors. In Mode 4 (304), thecommand voltage vector V* trajectory covers regions 10, 11, 12, 13, 14,15, and 16 of all sectors.

Operational Modes

FIG. 4 is a space vector diagram illustrating a sector A triangle formedby voltage vectors V₀, V₁₀, and V₁₄. In Space Vector Pulse WidthModulation (SVPWM), the inverter voltage vectors V₃, V₆, and V₇, whichcorrespond to the apexes of the region 7 of the sector A triangle, whichincludes the reference or command voltage vector (V*) 412, are generallyselected to minimize harmonics at the output of the multi-levelinverter. If the command voltage vector V* lies in region 7 (422), asshown in FIG. 4, the following two equations are satisfied for SVPWM:V ₆ T _(a) +V ₃ T _(b) +T _(c) =V*T _(S)  (1)T _(a) +T _(b) +T _(c) =T _(S)  (2)where T_(a), T_(b), and T_(c) are respective time intervals of thenearest three voltage vectors in a particular triangle, and T_(S) is thesampling time.

FIG. 5 is a space vector diagram illustrating switching states andswitching times of the three phases of region 1 and sectors A-F in Mode1.

In embodiments, the switching sequence is pre-defined (and may be storedin a look-up table in memory) and depends on the location of referencevoltage vector (V*) in any particular region or triangle. The sequencein opposite sectors, e.g., A-D, B-E, and C-F, is selected to be of acomplimentary nature to achieve capacitor neutral voltage balancing.

FIG. 6 is a waveform diagram showing the construction of AC voltagewaveform patterns based on the sequences of switching states (P₂, P₁, O,N₁, N₂) of the U phase in region 1 of sectors A-F (U_(A1)-U_(F1)) forMode 1 operation. The sequences of switching states are obtained fromthe space vector diagram of FIG. 5 where the switching states for thethree phases are defined at each of the voltage vectors. The first row501 of switching states for voltage vector V₀ are switching states forthe U phase, the second row 502 of switching states for voltage vectorV₀ are switching states for the V phase, and the third row 503 ofswitching states for voltage vector V₀ are switching states for the Wphase. Likewise, the first row 511 of switching states for voltagevector V₁ are switching states for the U phase, the second row 512 ofswitching states for voltage vector V₁ are switching states for the Vphase, and the third row 513 of switching states for voltage vector V₁are switching states for the W phase. Further, the first row 521 ofswitching states for voltage vector V₄ are switching states for the Uphase, the second row 522 of switching states for voltage vector V₄ areswitching states for the V phase, and the third row 523 of switchingstates for voltage vector V₄ are switching states for the W phase.

To construct the first half of the sequence of switching states forphase U, sector A, region 1 (U_(A1)) (611 a) over sampling period Ts/2(601), switching states are obtained from each of the voltage vectors ina counter-clockwise direction. The switching states are obtained fromright to left in the first row of switching states assigned to each ofthe voltage vectors.

For example, the first switching state of the sequence U_(A1) is theright-most switching state in the first row 501 for voltage vector V₀,which is N₂. The second switching state of the sequence U_(A1) is theright-most switching state in the first row 511 for voltage vector V₁,which is N₁. The third switching state of the sequence U_(A1) is theright-most switching state in the first row 521 for voltage vector V₄,which is N₁. The fourth switching state of the sequence U_(A1) is thesecond switching state from the right in the first row 501 for voltagevector V₀, which is N₁. The fifth switching state of the sequence U_(A1)is the second switching state from the right in the first row 511 forvoltage vector V₁, which is O. The sixth switching state of the sequenceU_(A1) is the second switching state from the right in the first row 521for voltage vector V₄, which is O. The seventh switching state of thesequence U_(A1) is the third switching state from the right in the firstrow 501 for voltage vector V₀, which is O.

The eighth switching state of the sequence U_(A1) is the third switchingstate from the right in the first row 511 for voltage vector V₁, whichis P₁. The ninth switching state of the sequence U_(A1) is the thirdswitching state from the right in the first row 511 for voltage vectorV₁, which is P₁. The tenth switching state of the sequence U_(A1) is thethird switching state from the right in the first row 521 for voltagevector V₄, which is P₁. The eleventh switching state of the sequenceU_(A1) is the fourth switching state from the right in the first row 501for voltage vector V₀, which is P₁.

The twelfth switching state of the sequence U_(A1) is the fourthswitching state from the right in the first row 511 for voltage vectorV₁, which is P₂. The thirteenth switching state of the sequence U_(A1)is the fourth switching state from the right in the first row 521 forvoltage vector V₄, which is P₂. The fourteenth switching state of thesequence U_(A1) is the fifth switching state from the right in the firstrow 501 for voltage vector V₀, which is P₂.

To construct the second half of the sequence of switching states forphase U, sector A, region 1 (U_(A1)) (611 b) over sampling period Ts/2(602), switching states are obtained from each of the voltage vectors ina clockwise direction. The switching states are obtained from left toright in the first row of switching states assigned to the voltagevectors.

FIG. 7 is a waveform diagram illustrating the construction of AC voltagewaveform patterns based on the vector space diagram of FIGS. 3A and 3Bin a manner similar to that described above with respect to FIG. 6. FIG.7 shows AC voltage waveform patterns for a sequence of switching states,e.g., P₂, P₁, O, N₁, N₂, of the three phases U, V, and W in regions 1-4of sector A for operational modes 1 and 2. The AC voltage waveformpatterns include waveform patterns U_(A1) (611), V_(A1) (712), andW_(A1) (713), which were constructed based on the switching statesobtained from region 1 in sector A.

Determination of Turn-on Times

PWM waveforms are established once switching turn-on time information isdetermined based on the following equations.

The turn-on time (T) is the sum function of weighted duty cycles T_(a),T_(b), and T_(c). Turn-on time T can be represented by the followingequation:T=f(K _(T-ON) of (T _(a) ,T _(b), and T _(a))),  (3)where K_(T-ON) is a coefficient of time-weighted duty cycles ofswitching times T_(a), T_(b), and T_(c). Switching times T_(a), T_(b),and T_(c) may be determined based on the ‘average value’ principle,which simplifies the implementation.

FIG. 8 is a waveform diagram illustrating generation of turn-on timesignals (U_(A1P2), U_(A1P1), U_(A1O), U_(A1N1)) and switching logicsignals (S_(UA1P2), S_(UA1P1), S_(UA1O), S_(UA1N1), S_(UA1N2)) ofrespective P₂, P₁, O, N₁, N₂ voltage levels for waveform U_(A1) ofsector A and region 1. As shown in FIG. 8, the switching pattern duringthe first T_(S)/2 interval is repeated inversely in the next T_(S)/2interval with appropriate segmentation of T_(a), T_(b), and T_(c).

Capacitor voltage balancing of multi-level diode-clamped voltage sourceinverters (VSI) of STATCOM and MVUPS is an issue as it supplies orabsorbs both active and reactive power. Capacitor voltage balancingbecomes more difficult as the numbers of capacitor to be balanced isincreased, e.g., from two (for three-level) to four (for five-level).Thus, switching sequences in opposite sectors (viz., A-D, B-E, and C-F)are selected to be of a complimentary nature to achieve capacitorneutral voltage balancing. The time interval duty cycles T_(a), T_(b),T_(c) are distributed appropriately so as to generate symmetrical PWMwaves with capacitor neutral point voltage balancing.

The turn-on time T₁ to establish the turn-on time signal (U_(A1P2)) ofthe P₂ voltage level is calculated as follows:T ₁ =K _(T-ON1-a-A1)(T _(a))K _(T-ON1-b-A1)(T _(b))K _(T-ON1-c-A1)(T_(a)),  (3a)where K_(T-ON1-a-A1) is a coefficient of time-weighted duty cycle T_(a),K_(T-ON1-b-A1) is a coefficient of time-weighted duty cycle T_(b), andK_(T-ON1-c-A1) is a coefficient of time-weighted duty cycle T_(c) ofSector A and Region 1.

The K_(T-ON1-a-A1) value may be calculated as ⅜ (=⅛+⅛+⅛), theK_(T-ON1-b-A1) value may be calculated as ⅖ (= 1/10+ 1/10+ 1/10+ 1/10),and the K_(T-ON1-c-A1) value may be calculated as ⅜ (=⅛+⅛+⅛), as shownin FIG. 8.

Switching times (T_(a), T_(b), and T_(c)) are determined based on the‘average value’ principle. Therefore,T ₁=⅜*T _(S)/3+⅖*T _(S)/3+⅜*T _(S)/3=0.76*T _(S)/2  (3a1)

The turn-on time T₂ to establish the turn-on time signal (U_(A1P1)) ofthe P₁ voltage level is calculated as follows:T ₂ =K _(T-ON2-a-A1)(T _(a))+K _(T-ON2-b-A1)(T _(b))+K _(T-ON2-c-A1)(T_(c)),  (3b)where K_(T-ON2-a-A1) is a coefficient of time-weighted duty cycle T_(a),K_(T-ON2-b-A1) is a coefficient of time-weighted duty cycle T_(b), andK_(T-ON2-c-A1) is a coefficient of time-weighted duty cycle T_(c) ofSector A and Region 1.

The K_(T-ON2-a-A1) value may be calculated as ¼ (=⅛+⅛), theK_(T-ON2-b-A1) value may be calculated as 3/10 (= 1/10+ 1/10+ 1/10), andthe K_(T-ON2-c-A1) value may be calculated as ¼ (=⅛+⅛), as shown in FIG.8.

Switching times (T_(a), T_(b), and T_(c)) are determined based on the‘average value’ principle. Therefore,T ₂=¼*T _(S)/3+ 3/10*T _(S)/3+¼*T _(S)/3=0.53*T _(S)/2.  (3b1)Similarly, T₃ for U_(A1O) of the O voltage level and T₄ for U_(A1N1) ofthe N₁ voltage level are determined for waveform U_(A1) of Sector A andRegion 1.

After the turn-on time values are calculated, they may be stored inmemory and used to generate the switching logic signals. As shown inFIG. 8, an up/down counter signal 805 starts below switching time T₄(e.g., time equal to zero), increases through switching times T₄-T₁before reaching T_(S)/2, then, after reaching T_(S)/2, decreases throughswitching times T₁-T₄. Switching times T₁-T₄ are compared to the up/downcounter signal 805, e.g., using comparators 1402-1408, respectively ofFIG. 14, to obtain the U_(A1P2) signal 810, the U_(A1P1) signal 814, theU_(A1O) signal 820, and the U_(A1N1) signal 826. The U_(A1P2) signal 810is used as the switching signal S_(UA1P2). The U_(A1P2) signal 810, theU_(A1P1) signal 814, the U_(A1O) signal 820, and the U_(A1N1) signal826, are then inverted, e.g., by the inverters 1412-1418, respectivelyof FIG. 14, to obtain the IU_(A1P2) signal 812, the IU_(A1P1) signal818, the IU_(A1O) signal 824, and the IU_(A1N1) signal 830. TheIU_(A1N1) signal 830 is used as the switching signal S_(UA1N1). Then,the Boolean AND operation is performed on the IU_(A1P2) signal 812 andthe U_(A1P1) signal 814, e.g., using the AND gate 1422 of FIG. 14, toobtain the S_(UA1P1) switching signal 816. The Boolean AND operation isalso performed on the IU_(A1P1) signal 818 and the U_(A1O) signal 820,e.g., using the AND gate 1424 of FIG. 14, to obtain the S_(UA1O)switching signal 822. The Boolean AND operation is further performed onthe IU_(A1O) signal 824 and the U_(A1N1) signal 826, e.g., using the ANDgate 1426 of FIG. 14, to obtain the S_(UA1N1) switching signal 828.

FIG. 9 is a waveform diagram illustrating the generation of turn-on timesignals (U_(B1N2), U_(B1N1), U_(B1O), U_(B1P1)) and switching logicsignals (S_(UB1N2), S_(UB1N1), S_(UB1O), S_(UB1P1), S_(UB1P2)) ofrespective N₂, N₁, O, P₁, P₂ voltage levels for waveform U_(B1) ofsector B and Region 1. As shown in FIG. 9, the switching pattern duringthe first T_(S)/2 interval 601 is repeated inversely in the next T_(S)/2interval 602 with appropriate segmentation of T_(a), T_(b), and T_(c).

The turn-on time value T₁ to establish the turn-on time signal(U_(B1N2)) of N₂ voltage level may be calculated as follows:T ₁ =K _(T-ON1-a-B1)(T _(a))+K _(T-ON1-b-B1)(T _(b))+K _(T-ON1-c-B1)(T_(a)),  (4a)where K_(T-ON1-a-B1) is a coefficient of the time-weighted duty cycleT_(a), K_(T-ON1-b-B1) is a coefficient of the time-weighted duty cycleT_(b), and K_(T-ON1-c-B1) is a coefficient of time-weighted duty cycleT_(c) of Sector B and Region 1.

The K_(T-ON1-a-B1) value may be calculated as ⅜ (=⅛+⅛+⅛), theK_(T-ON1-b-B1) value may be calculated as ⅖ (= 1/10+ 1/10+ 1/10+ 1/10),and the K_(T-ON1-c-B1) value may be calculated as (⅛+⅛+⅛+⅛=) ½ as shownin FIG. 9.

Switching times T_(a), T_(b), and T_(c) are determined based on theaverage value principle. Therefore,T ₁=⅜*T _(S)/3+⅖*T _(S)/3+½*T _(S)/3=0.85*T _(S)/2  (4a1)

The turn-on time value T₂ to establish turn-on time signal U_(B1N1) ofN₁ voltage level is calculated as follows:T ₂ =K _(T-ON2-a-B1)(T _(a))+K _(T-ON2-b-B1)(T _(b))K _(T-ON2-c-B1)(T_(a)),  (4b)where K_(T-ON2-a-B1) is a coefficient of time-weighted duty cycle T_(a),K_(T-ON2-b-B1) is a coefficient of time-weighted duty cycle T_(b), andK_(T-ON2-c-B1) is a coefficient of time-weighted duty cycle T_(c) ofSector B and Region 1.

The K_(T-ON2-a-B1) value may be calculated as ¼ (=⅛+⅛), theK_(T-ON2-b-B1) value may be calculated as 3/10 (= 1/10+ 1/10+ 1/10) andthe K_(T-ON2-c-B1) value may be calculated as ⅜ (=⅛+⅛+⅛) as shown inFIG. 9.

Switching times T_(a), T_(b), and T_(c) are determined based on theaverage value principle. Therefore,T ₂=¼*T _(S)/3+ 3/10*T _(S)/3+⅜*T _(S)/3=0.61*T _(S)/2.  (4b1)

Similarly, T₃ for U_(B1O) of the O voltage level and T₄ for U_(B1P1) ofthe P₁ voltage are determined for waveform U_(B1) of Sector B and Region1.

After the turn-on time values are calculated, they may be stored inmemory and used to generate the switching logic signals. As shown inFIG. 9, the up/down counter signal 805 starts below switching time T₄(e.g., a time equal to zero), increases through switching times T₄-T₁before reaching T_(S)/2, then, after reaching T_(S)/2, decreases throughswitching times T₁-T₄. Switching times T₁-T₄ are compared to the up/downcounter signal 805, e.g., using comparators 1402-1408, respectively ofFIG. 14, to obtain the U_(B1N2) signal 910, the U_(B1N1) signal 914, theU_(B1O) signal 920, and the U_(B1P1) signal 926. The U_(B1N2) signal 910is used as the switching signal S_(UB1N2).

The U_(B1N2) signal 910, the U_(B1N1) signal 914, the U_(B1O) signal920, and the U_(B1P1) signal 926, are then inverted, e.g., by theinverters 1412-1418, respectively of FIG. 14, to obtain the IU_(B1N2)signal 912, the IU_(B1N1) signal 918, the IU_(B1O) signal 924, and theIU_(B1P1) signal 930. The IU_(B1P1) signal 930 is used as the switchingsignal S_(UB1P1). Then, the Boolean AND operation is performed on theIU_(B1N2) signal 912 and the U_(B1N1) signal 914, e.g., using the ANDgate 1422 of FIG. 14, to obtain the S_(UB1N1) switching signal 916. TheBoolean AND operation is also performed on the IU_(B1N1) signal 918 andthe U_(B1O) signal 920, e.g., using the AND gate 1424 of FIG. 14, toobtain the S_(UB1O) switching signal 922. The Boolean AND operation isfurther performed on the IU_(B1O) signal 924 and the U_(B1P1) signal926, e.g., using the AND gate 1426 of FIG. 14, to obtain the S_(UB1P1)switching signal 928.

FIG. 10 is a waveform diagram illustrating the generation of turn-ontime signals U_(A2P2), U_(A2P1), U_(A2O) and switching logic signalsS_(UA2P2), S_(UA2P1), S_(UA2O), S_(UA2N1) of the respective P₂, P₁, O,N₁ voltage level for waveform U of Sector A and Region 2.

As shown in FIG. 10, the up/down counter signal 805 starts belowswitching time T₃, increases through switching times T₃-T₁ beforereaching T_(S)/2, then, after reaching T_(S)/2, decreases throughswitching times T₁-T₃. Switching times T₁-T₃ are compared to the up/downcounter signal 805, e.g., using comparators 1402-1406, respectively ofFIG. 14, to obtain the U_(A2P2) signal 1010, the U_(A2P1) signal 1014,and the U_(A2O) signal 1020. The U_(A2P2) signal 1010 is used as theswitching signal S_(UA2P2).

The U_(A2P2) signal 1010, the U_(A2P1) signal 1014, and the U_(A2O)signal 1020, are then inverted, e.g., by the inverters 1412-1416,respectively of FIG. 14, to obtain the IU_(A2P2) signal 1012, theIU_(A2P1) signal 1018, and the IU_(A2O) signal 1024. The IU_(A2O) signal1024 is used as the switching signal S_(UA2N1). Then, the Boolean ANDoperation is performed on the IU_(A2P2) signal 1012 and the U_(A2P1)signal 1014, e.g., using the AND gate 1422 of FIG. 14, to obtain theS_(UA2P1) switching signal 1016. The Boolean AND operation is alsoperformed on the IU_(A2P1) signal 1018 and the U_(A2O) signal 1020,e.g., using the AND gate 1424 of FIG. 14, to obtain the S_(UA2O)switching signal 1022.

FIG. 11 is a waveform diagram illustrating the generation of turn-ontime signals U_(A7P2), U_(A7P1) and switching logic signals S_(UA7P2),S_(UA7P1), S_(UA7O) of the respective P₂, P₁, O voltage level forwaveform U_(A7) of Sector A and Region 7.

As shown in FIG. 11, the up/down counter signal 805 starts belowswitching time T₂, increases through switching times T₂ and T₁ beforereaching T_(S)/2, then, after reaching T_(S)/2, decreases throughswitching times T₁ and T₂. Switching times T₁ and T₂ are compared to theup/down counter signal 805, e.g., using comparators 1402 and 1404,respectively of FIG. 14, to obtain the U_(A7P2) signal 1110 and theU_(A7P1) signal 1114. The U_(A7P2) signal 1110 is used as the switchingsignal S_(UA7P2).

The U_(A7P2) signal 1110 and the U_(A7P1) signal 1114 are then inverted,e.g., by the inverters 1412 and 1414, respectively of FIG. 14, to obtainthe IU_(A7P2) signal 1112 and the IU_(A7P1) signal 1118. The IU_(A7P1)signal 1118 is used as the switching signal S_(UA7O). Then, the BooleanAND operation is performed on the IU_(A7P2) signal 1112 and the U_(A7P1)signal 1114, e.g., using the AND gate 1422 of FIG. 14, to obtain theS_(UA7P1) switching signal 1116.

FIG. 12 is a waveform diagram illustrating the generation of turn-ontime signal U_(A13P2) and switching logic signals (S_(UA13P2),S_(UA13P1)) of the respective P₂, P₁ voltage level for waveform U_(AB)of Sector A and Region 13.

As shown in FIG. 12, the up/down counter signal 805 starts belowswitching time T₁ (e.g., time equal to zero), increases throughswitching time T₁ before reaching T_(S)/2, then, after reaching T_(S)/2,decreases through switching time T₁. Switching time T₁ is compared tothe up/down counter signal 805, e.g., using comparator 1402, to obtainthe U_(A13P2) signal 1210, which is used as the switching signalS_(UA13P2). The U_(A13P2) signal 1210 is then inverted, e.g., by theinverter 1412 of FIG. 14, to obtain the IU_(A13P2) signal 1212, which isused as the switching signal S_(UA13P1).

FIG. 13 is a graphical diagram illustrating example turn-on time valuesT₁, T₂, T₃, T₄ for phase U of all six sectors (Sectors A-F) and Region 1based on the equations set forth above. As shown, the turn-on timevalues T₁, T₂, T₃, T₄ are given for the switching states of each of thesectors. For phases V and W, the waveforms are similar to the waveformsin FIGS. 8-12, but are mutually phase shifted by the angle 2π/3.

PWM Signal Generation

FIG. 14 is a digital logic circuit for generating PWM switching signalsfor a five-level inverter. The digital logic circuit includes a firstcomparator 1402, a second comparator 1404, a third comparator 1406, afourth comparator 1408, a first inverter 1412, a second inverter 1414, athird inverter 1416, a fourth inverter 1418, a first AND gate 1422, asecond AND gate 1424, and a third AND gate 1426. The inverters 1412-1418are digital logic inverters and are connected to outputs of thecomparators 1402-1408, respectively. The AND gate 1422 has a first inputconnected to the output of the first inverter 1412 and a second inputconnected to the output of the second comparator 1404, the AND gate 1424has a first input connected to the output of the second inverter 1414and a second input connected to the output of the third comparator 1406;and the AND gate 1426 has a first input connected to the output of thethird inverter 1416 and a second input connected to the output of thefourth comparator 1408.

The first inputs of the comparators 1402-1408 receive turn-on timevalues T₁, T₂, T₃, and T₄, respectively, and the second inputs of thecomparators 1402-1408 receive the output from an up/down counter 1401.The up/down counter 1401 counts over a sampling period T_(S) from 0 toT_(S)/2 and from T_(S)/2 to 0. The turn-on time values T₁, T₂, T₃, andT₄ are compared with the output of the up/down counter 1401 usingdigital logic comparators 1402-1408 to generate turn-on pulse signalsU_(A/C/EP2), U_(A/C/EP1), U_(A/C/EO), U_(A/C/EN1) for A/C/E sectors orU_(B/D/FN2), U_(B/D/FN1), U_(B/D/FO), U_(B/D/FP1) for B/D/F sectors.These turn-on pulse signals are then logically inverted with multipleinverters 1412-1418 and logically ANDed by AND gates 1422-1426 togenerate switching logic signals S_(UA/C/EP2), S_(UA/C/EP1),S_(UA/C/EO), S_(UA/C/EN1), S_(UA/C/EN2) for A/C/E sectors orS_(UB/D/FN2), S_(UB/D/FN1), S_(UB/D/FO), S_(UB/D/FP1), S_(UB/D/FP2) forB/D/F sectors for all sectors and regions. The comparators 1402-1408,the inverters 1412-1418, and the AND gates 1422-1426 may be implementedby a Field Programmable Gate Array (FPGA) or an Application SpecificIntegrated Circuit (ASIC) for high-bandwidth fast operation.

In embodiments, the number of comparators, inverters, and AND gates maybe increased or decreased depending on the number of levels of themulti-level inverter.

As shown in FIG. 8, turn-on time values T₁, T₂, T₃, T₄ are used togenerate switching logic signals S_(UA1P2), S_(UA1P1), S_(UA1O),S_(UA1N1), S_(UA1N2). These switching logic signals are applied to gatedrivers that drive respective power transistors, such as IGBT devices,to generate the U_(A1) waveform of Phase U. Turn-on time values T₁, T₂,T₃, T₄ are used to generate switching logic signals for P₂, P₁, O, N₁,N₂ in that respective order for all A, C, and E odd sectors.

As shown in FIG. 9, turn-on time values T₁, T₂, T₃, T₄ are used togenerate switching logic signals S_(UB1N2), S_(UB1N1), S_(UB1O),S_(UB1P1), S_(UB1P2). These switching logic signals are applied to gatedrivers to drive respective power transistors to generate the U_(B1)waveform of Phase U in sector B of region 1. Turn-on time values T₁, T₂,T₃, T₄ are used to generate the switching logic signal for N₂, N₁, O,P₁, P₂ in that respective order for all B, D, and F even sectors.

Similar signal processing is done for all other regions and V and Wphases using the digital logic circuit. The turn-on time values T₁, T₂,T₃, and T₄ are different for Phases V and W. Some portion of the samecontrol circuit is used to generate switching logic signals based on thelocation of the voltage vector V* as shown in FIGS. 10, 11, and 12.

In embodiments, a single timer, counting from 0 to T_(S)/2 and then backto 0, and one digital logic circuit is used for all Sectors and Regions.Therefore, the complexity of SVPWM is simplified using the controlsystem according to the present disclosure.

Region and Sector Identification of Five-Level Inverter

FIG. 15 is a flow diagram of a method for determining region and sectorlocations of command voltage V*. In the beginning, the command orreference voltage V* and angle θ_(e)* are sampled. In step 1502, thecurrent sector location is identified based on the angle θ_(e)*. Foreasy implementation and calculation, all angles θ_(e)* located insectors B-F are remapped to sector A (θ_(eA)) according to Table 2below. Then, the command voltage V* and remapped angle θ_(eA) areconverted into X and Y voltage vector coordinate point values. The X andY voltage vector coordinate points are compared with a respectiveXY-line equation in a sector to determine the region.

TABLE 2 Θ_(e)* Location Sector Θ_(eA) 0 ≧ Θ_(e)* < π/3 A = Θ_(e)* π/3 ≧Θ_(e)* < 2*π/3 B = (Θ_(e)* − π/3) 2* π/3 ≧ Θ_(e)* < π C = (Θ_(e)* −2*π/3) π ≧ Θ_(e)* < 4*π/3 D = (Θ_(e)* − π) 4*π/3 ≧ Θ_(e)* < E = (Θ_(e)*− 4*π/3) 5*π/3 5*π/3 ≧ Θ_(e)* < 2* π F = (Θ_(e)* − 5*π/3)

For example, as shown in FIG. 15, in step 1504, it is determined whetherthe X-Y coordinate points are less than or equal to Y=−1.732X+0.866. Ifthe result of the determination in step 1504 is false, it is determinedwhether the X-Y coordinate points are greater than or equal toY=−1.732X−0.866 and the X coordinate point is greater than or equal to0.5, in step 1505. If the result of the determination in step 1505 istrue, it is determined in step 1506 that the voltage vector V* islocated in regions 5, 10, 11, or 12.

In step 1508, it is determined whether the X-Y coordinate points areless than or equal to Y=−1.732X+1.299. If the result of thedetermination in step 1508 is true, the voltage vector is determined tobe in region 5, in step 1510. If the result of the determination in step1508 is false, it is determined whether the X-Y coordinate points aregreater than or equal to Y=−1.732X−1.299 and X is greater than or equalto 0.75, in step 1512. If the result of the determination in step 1512is true, the voltage vector is determined to be in region 10, in step1514. If the result of the determination in step 1512 is false, it isdetermined whether the Y coordinate point is greater than or equal toY=0.2165, in step 1516. If the result of the determination in step 1516is true, the voltage vector is determined to be in region 12, in step1518. If the result of the determination in step 1516 is false, thevoltage vector is determined to be in region 11, in step 1520.

If the result of the determination in step 1505 is false, it isdetermined whether the Y coordinate point is greater than or equal toY=0.433 in step 1522. If the result of the determination in step 1522 istrue, it is determined in step 1524 that the voltage vector V* islocated in regions 9, 14, 15, or 16. In step 1526, it is determinedwhether the X-Y coordinate points are greater than or equal toY=1.732X−0.433. If the result of the determination in step 1526 is true,the voltage vector is determined to be in region 14, in step 1528. Ifthe result of the determination in step 1526 is false, it is determinedwhether the X-Y coordinate points are less than or equal toY=−1.732X+1.299, in step 1530. If the result of the determination instep 1530 is true, the voltage vector is determined to be in region 9,in step 1532. If the result of the determination in step 1530 is false,it is determined whether the Y coordinate point is greater than or equalto Y=0.6495, in step 1534. If the result of the determination in step1534 is true, the voltage vector is determined to be in region 16, instep 1536. If the result of the determination in step 1534 is false, thevoltage vector is determined to be in region 15, in step 1538.

If the result of the determination in step 1522 is false, it isdetermined in step 1540 that the voltage vector V* is located in regions6, 7, 8, or 13. In step 1542, it is determined whether the X-Ycoordinate points are less than or equal to Y=1.732X−0.433. If theresult of the determination in step 1542 is true, the voltage vector isdetermined to be in region 8, in step 1544. If the result of thedetermination in step 1542 is false, it is determined whether the X-Ycoordinate points are greater than or equal to Y=−1.732X+1.299, in step1546. If the result of the determination in step 1546 is true, thevoltage vector is determined to be in region 13, in step 1548. If theresult of the determination in step 1546 is false, it is determinedwhether the Y coordinate point is less than or equal to Y=0.2165, instep 1550. If the result of the determination in step 1550 is true, thevoltage vector is determined to be in region 6, in step 1552. If theresult of the determination in step 1550 is false, the voltage vector isdetermined to be in region 7, in step 1554.

If the result of the determination in step 1504 is true, it isdetermined in step 1556 that the voltage vector V* is located in regions1, 2, 3, or 4. In step 1558, it is determined whether the X-Y coordinatepoints are less than or equal to Y=−1.732X+0.433. If the result of thedetermination in step 1558 is true, the voltage vector is determined tobe in region 1, in step 1560. If the result of the determination in step1558 is false, it is determined whether the X-Y coordinate points aregreater than or equal to Y=−1.732X−0.433 and the X coordinate point isgreater than or equal to 0.25, in step 1562. If the result of thedetermination in step 1562 is true, the voltage vector is determined tobe in region 2, in step 1564. If the result of the determination in step1562 is false, it is determined whether the Y coordinate point isgreater than or equal to Y=0.2165, in step 1566. If the result of thedetermination in step 1566 is true, the voltage vector is determined tobe in region 4, in step 1568. If the result of the determination in step1566 is false, the voltage vector is determined to be in region 3, instep 1570.

SVPWM Controller

Once turn-on time values T₁, T₂, T₃, T₄ have been calculated for all P₂,P₁, O, N₁, N₂ states of all phases, it is possible to evaluate them inreal time with the help of a DSP and establish the Space Vector PWMwaves with the help of FPGA-based single timer and single digital logiccircuit as shown in FIG. 14.

FIG. 16 is a block diagram illustrating a control system for afive-level diode clamped inverter. The control system includes aprocessor, such as a digital signal processor (DSP), which is used toimplement blocks 1602 and 1604. Blocks 1602 and 1604 are easilyimplemented to carry out the SVPWM control strategy.

In block 1602, the processor samples a reference voltage vector V* andan angle θ_(e)*, and identifies a sector and region based on the sampledreference voltage vector V* and angle θ_(e)*. In block 1604, theprocessor selects predefined switching sequences and pre-calculatedturn-on signal values T₁, T₂, T₃, T₄ based on the identified sector andregion location of the reference voltage vector V*.

The control system also includes a digital logic circuit 1606, such asan FPGA, and an up/down counter for generating PWM switching signals.The digital logic circuit 1606, such as the digital logic circuit ofFIG. 14, generates PWM switching signals for the power transistordrivers based on the turn-on signal values T₁, T₂, T₃, T₄ and theselected switching sequence. In embodiments, less expensive DSPs andFPGAs may be used because the overall control implementation is dividedbetween a DSP and an FPGA circuit.

As described above, the overall control operation is divided intomultiple modes (e.g., Modes 1-4) in terms of sector and regionlocations. A single digital logic circuit 1606 is used to generateswitching logic signals for all the regions and sectors. This simplifiesthe control system and, in turn, reduces time to implement at minimumcost. Also, to simplify the control system, one up/down counter 1608(counting from 0 to T_(S)/2 and then from T_(S)/2 to 0) with samplingperiod T_(S) may be utilized.

FIG. 17 is a flow diagram of a method of controlling a multi-levelinverter according to embodiments of the present disclosure. Afterstarting in step 1701, a sector location is identified based on anactual angle of a reference voltage vector, in step 1702. In step 1704,the actual angle is converted into a converted angle located in a firstsector and, in step 1706, a region location is identified based on themagnitude of the reference voltage vector and the converted angle in thefirst sector. In step 1708, a switching sequence and a plurality ofturn-on signal values are selected based on the corresponding regionlocation in the actual sector of the reference voltage vector. In step1710, turn-on signal values are transmitted to second inputs of theplurality of comparators to generate switching signals for a powertransistor driver of a multi-level inverter. In step 1712, each of theplurality of turn-on signals are compared to a digital up/down countersignal to obtain a plurality of comparison signals including a firstcomparison signal and a second comparison signal. In step 1714, theplurality of comparison signals are inverted to obtain a plurality ofinverted signals including a first inverted signal. Then, before endingin step 1717, a logical AND operation is performed on the first invertedsignal and the second comparison signal to obtain a switching signal fora corresponding driver that drives a power transistor of a multi-levelinverter.

While several embodiments of the disclosure have been shown in thedrawings and/or discussed herein, it is not intended that the disclosurebe limited thereto, as it is intended that the disclosure be as broad inscope as the art will allow and that the specification be read likewise.Therefore, the above description should not be construed as limiting,but merely as exemplifications of particular embodiments. Those skilledin the art will envision other modifications within the scope and spiritof the claims appended hereto.

What is claimed is:
 1. A control system for a multi-level inverter,comprising: a digital logic circuit comprising: a plurality of digitallogic comparators including a first comparator and a second comparator;a plurality of inverters coupled to respective outputs of a respectiveplurality of comparators, the plurality of inverters including a firstinverter; and a plurality of AND gates including an AND gate, the ANDgate having a first input and a second input, the first input coupled tothe output of the first inverter and the second input coupled to theoutput of the second comparator; a digital up/down counter coupled tofirst inputs of the plurality of comparators, the up/down countercounting from 0 to Ts/2 and then from Ts/2 to 0, where Ts is a samplingperiod; and a processor and memory configured to: identify a sectorlocation based on an actual angle of a reference voltage vector; convertthe actual angle into a converted angle located in a first sector;identify a reference region location based on the magnitude of thereference voltage vector and the converted angle in the first sector;select a switching sequence and turn-on time values based on thecorresponding actual region location and actual sector; transmit turn-ontime values to second inputs of the plurality of comparators; andprovide the output of the first comparator, the outputs of the pluralityof AND gates, and the output of a last inverter of the plurality ofinverters as switching signals, which are transmitted to gate driversfor driving power transistors of the multi-level inverter.
 2. Thecontrol system of claim 1, wherein the logic circuit is a FieldProgrammable Gate Array (FPGA) or an Application Specific IntegratedCircuit (ASIC).
 3. The control system of claim 1, wherein the processorand memory are implemented by a digital signal processor (DSP).
 4. Thecontrol system of claim 1, wherein the number of the plurality ofcomparators and the number of the plurality of inverters is one lessthan the number of levels of the multi-level inverter.
 5. The controlsystem of claim 4, wherein the number of the plurality of AND gates isone less than the number of the plurality of comparators.
 6. The controlsystem of claim 1, wherein the processor and memory are furtherconfigured to: convert the reference voltage vector and the convertedangle into X and Y coordinate point values in first sector; and identifya region location based on the X and Y coordinate point values.
 7. Thecontrol system of claim 6, wherein the identifying a region locationincludes comparing the X and Y coordinate point values to segments oftriangles, which represent regions, in a vector space.
 8. The controlsystem of claim 6, wherein the turn-on time values and switchingsequence are predetermined for each sector and region, and stored in alook-up table stored in memory.
 9. The control system of claim 1,wherein the sector locations are sectors A-F and the first sector issector A.
 10. The control system of claim 1, wherein the multi-levelinverter is a five-level inverter, wherein the plurality of comparatorsfurther include a third comparator and a fourth comparator, wherein theplurality of inverters further include a second inverter, a thirdinverter, and a fourth inverter, wherein the plurality of AND gatesfurther include a second AND gate, and a third AND gate, wherein a firstinput of the second AND gate is coupled to the output of the secondinverter and a second input of the second AND gate is coupled to theoutput of the third comparator, wherein a first input of the third ANDgate is coupled to the output of the third inverter and a second inputof the third AND gate is coupled to the output of the fourth comparator,and wherein the output of the first comparator, the outputs of theplurality of AND gates, and the output of the fourth inverter providethe switching signals for an IGBT driver of a five-level inverter. 11.The control system of claim 1, wherein the multi-level inverter is afour-level inverter, wherein the plurality of comparators furtherinclude a third comparator, wherein the plurality of inverters furtherinclude a second inverter and a third inverter, wherein the plurality ofAND gates further include a second AND gate, wherein a first input ofthe second AND gate is coupled to the output of the second inverter anda second input of the second AND gate is coupled to the output of thethird comparator, wherein the output of the first comparator, theoutputs of the plurality of AND gates, and the output of the thirdinverter provide the switching signals.
 12. A method of controlling amulti-level inverter, comprising: identifying a sector location based onan actual angle of a reference voltage vector; converting the actualangle into a converted angle located in a first sector; identifying aregion location based on the magnitude of the reference voltage vectorand the converted angle in the first sector; selecting a switchingsequence and a plurality of turn-on time values based on thecorresponding region location in actual sector of reference voltagevector; transmitting the turn-on time values to second inputs of theplurality of comparators to generate switching signals, which aretransmitted to gate drivers for driving power transistors of themulti-level inverter; comparing each of the plurality of turn-on signalsto a digital up/down counter signal to obtain a plurality of comparisonsignals including a first comparison signal and a second comparisonsignal; inverting the plurality of comparison signals to obtain aplurality of inverted signals including a first inverted signal; andperforming a logical AND operation on the first inverted signal and thesecond comparison signal to obtain a switching signal, which istransmitted to a gate driver for driving a power transistor of themulti-level inverter.
 13. The method of claim 12, further comprising:converting the reference voltage vector and the converted angle into Xand Y coordinate point values; and identifying a region location basedon the X and Y coordinate point values.
 14. The method of claim 13,wherein the identifying a region location includes comparing the X and Ycoordinate point values to segments of triangles, which representregions, in a vector space.
 15. The method of claim 13, wherein theturn-on time values and switching sequences are predetermined for eachsector and region, and stored in a look-up table stored in memory. 16.The method of claim 12, wherein the first comparison signal is a firstswitching signal, wherein the logical AND operation is performed on thefirst inverted signal and the second comparison signal to obtain asecond switching signal, wherein the plurality of comparison signalsfurther include a third comparison signal and a fourth comparisonsignal, wherein the plurality of inverted signals further include asecond inverted signal, a third inverted signal, and a fourth invertedsignal, wherein the method further comprises: performing a secondlogical AND operation on the second inverted signal and the thirdcomparison signal to obtain a third switching signal; and performing asecond logical AND operation on the second inverted signal and the thirdcomparison signal to obtain a fourth switching signal, and wherein thefourth inverted signal is a fifth switching signal.
 17. The method ofclaim 16, wherein the first switching signal corresponds to either a P₂or N₂ switching state, wherein the second switching signal correspondsto either a P₁ or N₁ switching state, wherein the third switching signalcorresponds to an O switching state, wherein the fourth switching signalcorresponds to either an N₁ or P₁ switching state, and wherein the fifthswitching signal corresponds to either an N₂ or P₂ switching state. 18.An energy storage system comprising: an energy storage device; a DC-DCconverter coupled to the energy storage device; a multi-level invertercoupled to the DC-DC converter; and a controller for the multi-levelinverter, the controller comprising: a digital logic circuit comprising:a plurality of comparators including a first comparator and a secondcomparator; a plurality of inverters coupled to respective outputs of arespective plurality of comparators, the plurality of invertersincluding a first inverter; and a plurality of AND gates including afirst AND gate, the first AND gate having a first input and a secondinput, the first input coupled to the output of the first inverter andthe second input coupled to the output of the second comparator; acounter coupled to first inputs of the plurality of comparators; and aprocessor and memory configured to: identify a sector location based onan actual angle of a reference voltage vector; convert the actual angleinto a converted angle located in a first sector; identify a regionlocation based on the magnitude of the reference voltage vector and theconverted angle in the first sector; select a switching sequence andturn-on signal values based on the corresponding region and actualreference voltage vector location; transmit turn-on signal values tosecond inputs of the plurality of comparators; and provide the output ofthe first comparator, the outputs of the plurality of AND gates, and theoutput of a last inverter of the plurality of inverters as switchingsignals, which are transmitted to gate drivers for driving powertransistors of the multi-level inverter.